Напишите свою функцию для вывода имени пользователя и пропишите путь к ней в настройках
settings.py.
ACCOUNT_USER_DISPLAY (=a callable returning user.username)
A callable (or string of the form 'some.module.callable_name') that takes a user as its only argument and returns the display name of the user. The default implementation returns user.username.
Например:
ACCOUNT_USER_DISPLAY = lambda user: u'%s %s' % (user.first_name, user.last_name)
или
ACCOUNT_USER_DISPLAY = lambda user: user.get_full_name()
или
ACCOUNT_USER_DISPLAY = 'myapp.somemodule.utils.user_display'